In order to smoothly and safely realize movement or logistics, it is extremely important to ensure road traffic safety. In recent years, in order to prevent traffic accidents or the like, technological development or rule making for sophistication of safe driving support system have become active.
The safe driving support systems generally prevent accidents in advance by mutually communicating various pieces of information including positional information with automobiles, pedestrians, and road facilities. In the communication of the safe driving support system, a carrier sense multiple access/collision avoidance (CSMA/CA) system which is capable of controlling transmission timing in a decentralized manner without synchronization of each communication device is widely adopted.
However, in the CSMA/CA system, when there are many devices which try to perform communication, there is a problem that a plurality of devices transmits information at the same time such that information collision occurs and the success rate of communication decreases. In particular, when the success rate of communication in the safe driving support system decreases, information cannot be densely exchanged between vehicles and between pedestrians and the vehicle, and this is against the goal of preventing accidents. Here, in a communication device, congestion control is performed when congestion of a communication channel is detected, and congestion of communication channels is mitigated by, for example, lowering the transmission frequency of information, and thus, collision of information is prevented.
However, when the transmission frequency is merely lowered with respect to the increase in traffic volume, the timing at which positional information of other vehicles can be acquired is delayed, and there is a concern that driving support is not possible at an appropriate timing. In particular, when the vehicle is moving at high speed, or when sudden acceleration or sudden deceleration is performed, if the acquisition timing of the positional information is delayed, there is a problem that the error from the actual position of the vehicle becomes large. In order to solve the problem, there is a technology for adjusting the following transmission frequency in accordance with the change in traveling speed and behavior of the vehicle.
However, according to the technologies described in Patent Literature 1 and Non Patent Literature 1, in order to adjust the transmission frequency in accordance with the traveling state of the host vehicle, when the host vehicle is stopped or travels at a constant speed at low speed, there is a possibility that the timing at which the other approaching vehicle acquires the positional information of the host vehicle is delayed.
In addition, Patent Literature 2 and Patent Literature 3 adjust the transmission frequency in accordance with the traveling speed of the host vehicle and other vehicles, but when supporting a collision prevention against traffic congestion, in Patent Literature 2, since the speed difference between vehicles in the traffic congestion is small, the transmission frequency is lowered.
In addition, Patent Literature 3 is designed to immediately transmit information when there is a speed difference from the approaching vehicle. However, in this method, since congestion recurs when a large number of vehicles in traffic congestion detects an approaching vehicle, the information is transmitted in accordance with the transmission probability that corresponds to the number of vehicles at the end of the traffic congestion. However, when the transmission is performed according to the transmission probability, for example, there remains a possibility that a vehicle having a high risk which is positioned at the end does not transmit the positional information, and there is a concern that driving support cannot be performed at an appropriate timing with respect to the approaching vehicle.
In addition, when performing the detection only from the viewpoint of whether or not the vehicle is at the end of the traffic congestion, as disclosed in Patent Literature 3, for example, when a vehicle is stopped on the shoulder while the traffic lane is congested, since the stopped vehicle is not at the end and the transmission frequency is still low, there is a case where the discovery of such a vehicle is delayed. Otherwise, in situations other than traffic congestion, for example, when a straight traveling vehicle approaches a vehicle which waits to turn to the right at an intersection, or when a plurality of vehicles enter from different directions at an intersection, there is no concept about the end, and thus, it is difficult to respond to the situation.    [Patent Literature 1] JP-2006-165806-A    [Patent Literature 2] WO 2012/124685-A1    [Patent Literature 3] JP-2010-288141-A    [Non Patent Literature 1] ETSI EN302 637-2 V1.3.2